This invention relates generally to a sootblower device for directing a fluid spray against a heat exchanger surface for cleaning the heat exchanger surface and in particular to a sootblower device having variable translational and rotational speeds along with a variable discharge of blowing medium to optimize the application of the blowing medium against the heat exchange surfaces while decreasing the time for completion of the operating cycle.
Cleaning highly heated surfaces, such as the surfaces of a boiler, furnace, incinerators or the like used to extract heat, has commonly been performed by devices generally known as sootblowers. Sootblowers typically employ water, steam, air or a combination thereof as a blowing medium which is directed through one or more nozzles against encrustations of slag, ash, scale and/or other foul materials which become deposited on the heat exchanger surfaces. Throughout the specification claims, the term "heat exchanger" is broadly used to refer to boilers, furnaces, incinerators or the like having internal surfaces in need of periodic cleaning to remove encrustations.
It is known that water in liquid form, either used alone or in combination with a gaseous blowing medium, increases the ease with which the encrustations are dislodged. The effectiveness of water in dislodging the encrustations results from a thermal shock effect coupled with mechanical impact. The thermal shock shrinks and embrittles the encrustations resulting in a fracturing of the encrustations so that they become dislodged and fall away from the heat exchanger surfaces because of the mechanical impact.
Various types of sootblowers have been developed for cleaning heat exchanger surfaces. One type of sootblower is known as the retracting variety which employs a lance tube that is advanced into a heat exchanger through a wall port. The lance tube has one or more nozzles through which the cleaning or blowing medium is discharged and sprayed against the heat exchanger surfaces. After a cleaning cycle has been completed the lance tube is retracted from the heat exchanger until cleaning is again needed. During each cleaning cycle, in addition to being advanced and retracted into and from the boiler, the lance tube is often rotated so that the spray of blowing medium is directed along a spiral path against the heat exchanger surfaces. Retractable sootblowers are used in applications where the internal temperatures of the heat exchanger are sufficient to damage the lance tube and shorten its life if permanently installed in the heat exchanger. Other sootblowers employ a permanently positioned lance tubes which, during each cleaning cycle, may be rotated or rotationally oscillated back and forth to move the jet stream of the blowing medium.
Unfortunately, to obtain sufficient cleaning with the water spray process mentioned above, a danger of over stressing the hot heat exchanger surfaces is present. Rapid deterioration of the heat exchanger surfaces as a result of thermal shock from the cleaning process has been observed. The problem of heat exchanger surface deterioration has been particularly severe in connection with cleaning the rigidly held tube bundles of large scale boilers. Being rigidly held, the tubes can not readily distort in response to the temperature induced shrinkage and expansion occurring during a cleaning cycle. The potential for damage to the heat exchanger surfaces is greater if the blowing medium is sprayed against a surface a second time, after it has been recently cleaned, such that the blowing medium contacts the surface directly rather than contacting an encrustation on the surface. Such multiple cleanings of a surface can occur where the jet stream from two adjacent sootblowers overlap one another. As a result, during the cleaning portion of the operating cycle it is desirable to periodically terminate the flow of the blow medium from the sootblower where the jet stream will cover a previously cleaned surface.
During certain other portions of the cleaning cycle, the jet stream will not be directed toward a heat exchanger surface in need of cleaning. One such situation is during the retraction stroke of the lance tube when cleaning has been performed during the insertion stroke of the lance tube (or vice versa). In this situation, during the retraction stroke of the lance tube, the discharging of the blowing medium is not desirable since it results in unnecessary blowing medium consumption. It is also desirable to avoid the needless discharge of blowing medium into the heat exchanger since this places an unnecessary thermal load on the heat exchanger and decreases efficiency. Another situation encountered is where during rotation of the sootblower lance tube, the emitted jet of blowing medium strikes surfaces to be cleaned over one or more arcs which combined are less than a full rotation of the jet. In other words, the jet is not performing a cleaning function during medium discharge over one or more portions of the rotational movement of the sootblower lance would reduce unneeded blowing medium discharge.
In terminating or reducing the discharge of the blowing medium, however, it is not always possible or practical to entirely eliminate the flow of the blowing medium. For example, it may be necessary to maintain a minimum flow rate through the lance tube in order to provide cooling of the lance tube within the heat exchanger. This will result in less wear on the heat exchange surfaces.
In additional to controlling the rate of blowing medium discharge, controlling the speed of the lance tube (the rate at which the lance tube is inserted, retracted and rotated) both during cleaning and non-cleaning portions of the operating cycle, will optimize the time it takes to complete the operating cycle. This further limits wear on the heat exchange surfaces and the lance tube.
Accordingly, it is an object of the present invention to provide a means for regulating the flow of the blowing medium from the lance tube into the heat exchanger during each cleaning cycle, depending on whether the lance tube nozzles are directed toward a surface which has already been cleaned during that operating cycle, to reduce blowing medium consumption.
It is also an object of the present invention to control, over the course of the operating cycle, the speed at which the lance tube is inserted, retracted and rotated so as to reduce and optimize the duration of the operating cycle.
Another object of this invention is to provide a sootblower in which cleaning is performed solely during either the insertion or retraction stroke of the lance tube while blowing medium discharge and lance tube speed are controlled to optimize the efficiency of the operating cycle.
In one embodiment of the invention, the lance tube is equipped with an inner tube extending therein creating an inner passage within the inner tube and an outer passage between the inner tube and the inner surface of the lance tube. The outer passage is used for supplying the blowing medium to the lance tube while the inner passage is used for return of a portion of the blowing medium for discarding externally of the heat exchanger. By opening the return flow path, the flow or amount of blowing medium being discharged through the lance tube nozzles is controllable based on the relative restriction to flow of the blowing medium through the nozzle as compared to the return flow path. When it is desirable to terminate or at least reduce the flow of the blowing medium through the lance tube nozzles, such as when the discharge would result in double cleaning of a surface or would occur during a non-cleaning portion of the operating cycle, the supply of blowing medium can be reduced to a minimum value necessary for cooling and other purposes. However, to further reduce the discharge of blowing medium through the nozzle, the return flow path is open whereby only a portion of the blowing medium used for cooling, etc. is discharged through the nozzles and into the heat exchanger. The remainder is discharged externally of the heat exchanger.
In the present invention, the speed or rate at which the lance tube is inserted, retracted and/or rotated can be varied. For example, when the nozzles and jets are directed at an already cleaned surface, the rate of rotation and/or translation can be increased to reduce the overall operating cycle time while the discharge rate of the blowing medium is decreased to reduce consumption.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.